This invention relates generally to integrated circuits and in particular to field programmable logic arrays with transistors with vertical gates.
Logic circuits are an integral part of digital systems, such as computers. These devices present a problem to integrated circuit manufacturers, who cannot afford to make integrated logic circuits perfectly tailored to the specific needs of every customer. Instead, general purpose very large scale integration (VLSI) circuits are defined. VLSI circuits serve as many logic roles as possible, which helps to consolidate desired logic functions. However, random logic circuits are still required to tie the various elements of a digital system together.
Several schemes are used to implement these random logic circuits. One solution is standard logic, such as transistor-transistor logic (TTL). TTL integrated circuits are versatile because they integrate only a relatively small number of commonly used logic functions. The drawback is that large numbers of TTL integrated circuits are typically required for a specific application. This increases the consumption of power and board space, and drives up the overall cost of the digital system.
Other alternatives include fully custom logic integrated circuits and semi-custom logic integrated circuits, such as gate arrays. Custom logic circuits are precisely tailored to the needs of a specific application. This allows the implementation of specific circuit architectures that dramatically reduces the number of parts required for a system. However, custom logic devices require significantly greater engineering time and effort, which increases the cost to develop these circuits and may also delay the production of the end system.
Semi-custom gate arrays are less expensive to develop and offer faster turnaround because the circuits are typically identical except for a few final-stage steps, which are customized according to the system design specifically. However, semi-custom gate arrays are less dense, so that it takes more gate array circuits than custom circuits to implement a given amount of random logic.
Between the extremes of general purpose devices on the one hand and custom and semi-custom gate arrays on the other, are programmable logic arrays (PLAs). PLAs which are programmable out in the field are known as field programmable logic arrays (FPLAs). FPLAs provide a more flexible architecture via user-programmed on-chip fuses or switches to perform specific functions for a given application. FPLAs can be purchased xe2x80x9coff the shelfxe2x80x9d like standard logic gates and are custom tailored like gate arrays in a matter of minutes.
To use FPLAs, system designers draft equations describing how the hardware is to perform, and enter the equations into a FPLA programming machine. The unprogrammed FPLAs are inserted into the machine, which interprets the equations and provides appropriate signals to the device to program the FPLA which will perform the desired logic function in the user""s system.
Recently, FPLAs based on erasable-programmable-read-only memory cells (EPROMs) fabricated with CMOS (complimentary-metal-oxide-semiconductor) technology have been introduced. Such devices employ floating gate transistors as the FPLA switches, which are programmed by hot electron effects. The EPROM cells are erased by exposure to ultraviolet light or other means. EEPROMs (Electrically Erasable Programmable Read Only Memory) can be erased and programmed while in circuit using Fowler-Nordheim tunneling. However, a disadvantage of current EEPROMs is that they have a large cell size and require two transistors per cell. Herein is where one of the problem lies.
Technological advances have permitted semiconductor integrated circuits to comprise significantly more circuit elements in a given silicon area. To achieve higher population capacities, circuit designers strive to reduce the size of the individual circuit elements to maximize available die real estate. FPLAs are no different than the other circuit elements in that denser circuits are required to support these technological advances.
Another important problem with EEPROM, EAPROM electrically alterable Programable Read only Memory, and flash memory devices is the adverse capacitance ratio between the control gate and the floating gate. That is, the capacitance between the control gate to floating gate (CCG) is about the same as the floating gate to substrate capacitance (CFG). FIG. 1A is a block diagram of a horizontal EEPROM, EAPROM, or flash memory device formed according to the teachings of the prior art. As shown in FIG. 1A, conventional horizontal floating gate transistor structure 101 includes a source region 110 and a drain region 112 separated by a channel region 106 in a horizontal substrate 100. A floating gate 104 is separated by a thin tunnel gate oxide 105 shown with a thickness (t1). A control gate 102 is separated from the floating gate 104 by an intergate dielectric 103 shown with a thickness (t2). Such conventional devices must by necessity have a control gate 102 and a floating gate 104 which are about the same size in width.
FIG. 1B is a block diagram of a vertical EEPROM, EAPROM, or flash memory device formed according to the disclosure in a co-pending, commonly assigned application by W. Noble and L. Forbes, entitled xe2x80x9cField programmable logic array with vertical transistors,xe2x80x9d Ser. No. 09/032617 U.S. Pat. No. 6,124,729, filed Feb. 27, 1998. FIG. 1B illustrates that vertical floating gate transistor structures have a stacked source region 110 and drain region 112 separated by a vertical channel region 106. The vertical floating gate transistor shown in FIG. 1B further includes a vertical floating gate 104 separated by a thin tunnel gate oxide 105 from the channel region 106. A vertical control gate 102 is separated from the floating gate 104 by an intergate dielectric 103. As shown in FIG. 1B, the vertical control gate 102 and the vertical floating gate 104 are likewise about the same size in width relative to the channel region 106.
Conventionally, the insulator, or intergate dielectric, 103 between the control gate 102 and the floating gate 104 is thicker (t2) than the gate oxide 105 (t1) to avoid tunnel current between the gates. The insulator, or intergate dielectric, 103 is also generally made of a higher dielectric constant insulator 103, such as silicon nitride or silicon oxynitride. This greater insulator thickness (t2) tends to reduce capacitance. The higher dielectric constant insulator 103, on the other hand, increases capacitance. As shown in FIG. 1C, the net result is that the capacitance between the control gate and the floating gate (CCG) is about the same as the gate capacitance of the thinner gate tunneling oxide 105 between the floating gate and the substrate (CFG). This undesirably results in large control gate voltages being required for tunneling, since the floating gate potential will be only about one half that applied to the control gate.
As design rules and feature size (F) in floating gate transistors continue to shrink, the available chip surface space in which to fabricate the floating gate also is reduced. In order to achieve a higher capacitance between the control gate and floating gate (CCG) some devices have used even higher dielectric constant insulators between the control gate and floating gate. Unfortunately, using such higher dielectric constant insulators involves added costs and complexity to the fabrication process.
Therefore, there is a need in the art to provide field programmable logic arrays which can operate with lower control gate voltages and which do not increase the costs or complexity of the fabrication process. Further such devices should desirably be able to scale with shrinking design rules and feature sizes in order to provide even higher density integrated circuits.
The above mentioned problems with field programmable logic arrays (PLA""s) and other problems are addressed by the present invention and will be understood by reading and studying the following specification. Structures and methods for programmable logic arrays are provided with logic cells, or floating gate transistors, which can operate with lower applied control gate voltages than conventional programmable logic arrays. The programmable logic arrays of the present invention do not increase the costs or complexity of the fabrication process. These arrays and methods are fully scalable with shrinking design rules and feature sizes in order to provide even higher density integrated circuits. The total capacitance of the logic cells within the programmable logic arrays is about the same as that for the prior art of comparable source and drain spacings. However, according to the teachings of the present invention, the floating gate capacitance in the logic cells is much smaller than the control gate capacitance such that the majority of any voltage applied to the control gate will appear across the floating gate thin tunnel oxide. Thus, the logic cells in the PLA""s of the present invention can be programmed by tunneling of electrons to and from the silicon substrate at lower control gate voltages than is possible in the prior art.
In one embodiment of the present invention, a novel programmable logic array is provided. The PLA includes a plurality of input lines for receiving an input signal and a plurality of output lines. One or more arrays is included in the present invention including a first logic plane and a second logic plane connected between the input lines and the output lines. The first logic plane and the second logic plane include a plurality of logic cells arranged in rows and columns for providing a sum-of-products term on the output lines responsive to the received input signal. According to the teachings of the present invention each logic cell includes a source region and a drain region in a horizontal substrate separated by a channel region. A first vertical gate is located above a first portion of the channel region and separated from the channel region by a first thickness insulator material. A second vertical gate is located above another portion of the channel region and separated therefrom by a second thickness insulator material. According to the teachings of the present invention, the second vertical gate opposes the first vertical gate and is separated from the first vertical gate by an intergate dielectric.
These and other embodiments, aspects, advantages, and features of the present invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art by reference to the following description of the invention and referenced drawings or by practice of the invention. The aspects, advantages, and features of the invention are realized and attained by means of the instrumentalities, procedures, and combinations particularly pointed out in the appended claims.